If you are running Jaguar and have the developer tools installed, you can turn the metal look on and off for most any window.

First, make a copy of an app to experiment on. Click on the copy, choose "Show Package Contents" from the contextual menu, and navigate to /Contents -> Resources -> English.lproj (assuming you want to change the English version). Look through the "nib" files and find the one with the name describing the window you want to change, and double-click it. The main window for most apps is in the file MainMenu.nib.

The nib file will open in Interface Builder. Click the icon for the window you want to change. Hit Command-1 to bring up the inspector, and uncheck "Textured Window." Save your changes, restart your program, and presto! No more metal. I've tested this on Address Book and it worked as described.

You can also just install this haxie from Unsanity. All you need to do for it to de-metallify, instead of metallify, is to add the letters "De" to the beginning of the file in the "Application Enhancers" folder. (See its README file.)

That Haxie is great if you want to turn it all off, but this hint is awesome because I can pick which apps to turn off. I kinda like iChat with metal, but I'm looking forward to messing around with Address Book and maybe Sherlock.